Lithographic apparatus and device manufacturing method

ABSTRACT

Methods and apparatus for maintaining the thermal equilibrium of a substrate and an immersion lithographic apparatus are disclosed using or having a timetable comprising information regarding the position, speed and/or acceleration of the substrate; and an evaporation controller and/or condensation controller acting to decrease localized evaporation and/or increase localized condensation in response to the information in the timetable. Evaporation of liquid from the surface of the substrate cools it down, while condensing liquid on its bottom surface heats the substrate locally.

1. RELATED APPLICATIONS

This is non-provisional application claims benefit of and priority toU.S. Provisional Application No. 60/692,234, filed Jun. 21, 2005, theentire contents of which application is hereby incorporated byreference.

2. FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device. In particular, the present invention relatesto a lithographic apparatus comprising a liquid confinement systemconfigured to confine liquid between a projection system and a substrateto be patterned by the projection system within the lithographicapparatus.

3. BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent applicationpublication WO 99/49504, hereby incorporated in its entirety byreference. As illustrated in FIGS. 2 and 3, liquid is supplied by atleast one inlet IN onto the substrate, preferably along the direction ofmovement of the substrate relative to the final element, and is removedby at least one outlet OUT after having passed under the projectionsystem. That is, as the substrate is scanned beneath the element in a −Xdirection, liquid is supplied at the +X side of the element and taken upat the −X side. FIG. 2 shows the arrangement schematically in whichliquid is supplied via inlet IN and is taken up on the other side of theelement by outlet OUT which is connected to a low pressure source. Inthe illustration of FIG. 2 the liquid is supplied along the direction ofmovement of the substrate relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in-and out-lets positioned around the final element are possible, oneexample is illustrated in FIG. 3 in which four sets of an inlet with anoutlet on either side are provided in a regular pattern around the finalelement.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference, the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting a substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus has only one table.

A problem with having a liquid in an immersion system as described aboveis that there is the risk of evaporation of the immersion liquid. Theevaporation of the immersion liquid may have one or more consequences. Afirst is a possible problem of localized cooling of the substrate,resulting in the shrinkage of the substrate and thus overlay and focuserrors from bimetallic bending caused by differences in the coefficientof thermal expansion between the substrate and the substrate table. Evenif the substrate and the substrate table have the same coefficient ofthermal expansion, a temperature gradient in the z-plane may causebending and thereby possibly introduce focus errors. A second is thepossible presence of drying stains on the substrate surface. A thirdconsequence is the possible cooling of the liquid confinement system,which indirectly cools the projection system and may cause sphericalaberrations in the projected beam. Furthermore, a temperature changecaused by evaporation may cause the substrate table or the substrateholder sizes and shapes to change. The position stability and signals ofthe sensors on the substrate table may be prone to drifting if, forexample, the substrate table changes shape. Finally, indirect cooling ofthe immersion liquid by the cooled liquid confinement system may resultin refractive index variation in the liquid, which in turn may causefocus offsets and distortions in the projected beam and thus in thepattern.

The operation of the lithographic apparatus as a whole is statedependent and so any variation in energy in any of the parts of thesystem may lead to errors in the exposure of the resist layer on thesubstrate. The concentration difference in relative humidity just abovea liquid film on the substrate, substrate table, substrate holder andsensors may cause the liquid in the liquid film to evaporate, causingone or more (or other) of the problems outlined above.

There are several places in the immersion system where evaporation ofthe immersion liquid may occur. These are:

1. between the liquid confinement system and the substrate;

2. on the surface of the substrate after wetting by the liquidconfinement system;

3. in the exhaust channels inside the liquid confinement system; and/or

4. in a gutter or gap between the substrate holder and the substratetable containing the substrate holder.

Evaporation at these places may lead to temperature reduction andtherefore may lead to energy loss in or at the projection system, theimmersion fluid, the liquid confinement system, the sensors, thesubstrate, the substrate holder and the substrate table that containsthe substrate holder and the substrate.

There are one or more reasons why net evaporation would be greater thannet condensation, possibly causing a net loss of energy in the system. Afirst possible reason is the use of a gas knife, which is used to pushthe liquid film on the substrate back and drag it forward as thesubstrate moves in and out of the immersion system. The gas knife doesthis using a pressurized gas jet. Evaporation is a possible side effectof the movement of the liquid film. If liquid loss from the surface ofthe substrate is too large, the liquid film pushed by the gas knife istoo large and the gas knife collapses. If this occurs, the liquid filmevaporates outside the immersion system in the ambient environment.

A second possible reason is when the gas knife is not used or notcompletely effective, the substrate and substrate table are movedrelative to the liquid confinement structure, so that a portion of thesubstrate and substrate table will be exposed to the atmospheresurrounding the lithographic apparatus. Any liquid remaining on thesubstrate or substrate table is likely to evaporate into the atmosphere,thus reducing the energy of the system. Evaporation of liquid from theliquid supply or exhaust system cools down the entire liquid confinementsystem, which cools down the immersion liquid, thus reducing the energyof the system and causing one or more (or other) of the problemsdescribed above.

4. SUMMARY

It is desirable to provide a system and method that efficiently andeffectively eliminates the reduction of net energy in a liquidconfinement system of a lithographic apparatus.

According to an aspect of the present invention, there is provided alithographic apparatus configured to project a patterned radiation beamonto a target portion of a substrate held on a substrate holder of asubstrate table, comprising:

a liquid supply system configured to at least partly fill a spacebetween a projection system and the substrate table with liquid;

a liquid confinement structure arranged to at least partially containthe liquid within the space;

a substrate table displacement system arranged to move the substratetable along a path relative to the liquid confinement structure, therebyto move the target portion over the surface of the substrate;

a timetable comprising positions and timings of the substratedisplacement system at which the substrate is likely to be undergoinglocalized cooling; and

a liquid evaporation or condensation controller arranged to control thenet rate of energy lost through evaporation or condensation of liquidsupplied by the liquid supply system by synchronizing evaporation orcondensation controlling techniques with the timetable.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

supplying a liquid to a space between a projection system of alithographic apparatus and a substrate using a liquid supply system;

controlling a net rate of evaporation or condensation of liquid suppliedby the liquid supply system by synchronizing evaporation or condensationwith a timetable comprising positions and timings of the substrate atwhich the substrate is likely to be undergoing localized cooling; and

projecting a patterned radiation beam through the liquid onto a targetportion of the substrate.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

supplying a liquid to a space between a projection system of alithographic apparatus and a substrate held by a substrate table, theliquid at least partially contained by a liquid confinement structure;

controlling the rate of energy lost through evaporation of the liquid;

heating a portion of the substrate according to (i) the position, (ii)velocity, (iii) acceleration, (iv) path, or (v) any combination of(i)-(iv) of the substrate table relative to the liquid confinementstructure, the local substrate temperature or the local substrate tabletemperature; and

switching simultaneously between heating and controlling the rate ofenergy lost.

Further aspects of the invention include condensation principles,adsorption principles and phase change material principles as methods ofevaporation energy controlling techniques. Condensation principlescontrol the amount of vapor available to condense on a surface—thegreater the amount, the greater the heat energy is transferred to thesurface on which the condensation occurs. Adsorption principles occurduring the cooling down and heating up of materials—as a material coolsdown, gas is adsorbed into the material. Desorption of the gas occursduring unloading of the substrate so that in the subsequent substratecycle, the absorption process may start again. During cooling, the stateof a material changes, releasing energy. Regeneration of the phasechange material is required during the unloading of a substrate, so thatwith the next substrate cycle, the process may start again.

5. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system used in a prior artlithographic projection apparatus;

FIG. 4 depicts a liquid supply system according to another prior artlithographic projection apparatus;

FIG. 5 depicts a liquid confinement system according to an embodiment ofthe present invention;

FIG. 6 depicts a liquid evaporation controlling system according to afirst embodiment of the invention;

FIG. 7 depicts a liquid evaporation controlling system according to asecond embodiment of the invention;

FIG. 8 depicts an energy regulating system according to a thirdembodiment of the invention;

FIG. 9 depicts an energy regulating system according to a fourthembodiment of the invention;

FIG. 10 depicts an energy regulating system according to a fifthembodiment of the invention;

FIG. 11 depicts a temperature sensor according to a sixth embodiment ofthe invention; and

FIG. 12 depicts alternative positions for a temperature sensor accordingto further embodiments of the invention.

6. DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) L configured to condition a        radiation beam B (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. The liquid confinementstructure is substantially stationary relative to the projection systemin the XY plane though there may be some relative movement in the Zdirection (in the direction of the optical axis). In an embodiment, aseal is formed between the liquid confinement structure and the surfaceof the substrate. The seal may be a contactless seal such as a gas seal.

FIG. 5 shows an example of such a solution and depicts a liquidconfinement system comprising a reservoir configured to hold immersionliquid of the immersion system according to an embodiment of theinvention.

The reservoir 10 forms a contactless seal to the substrate around theimage field of the projection system so that liquid is confined to filla space between the substrate surface and the final element of theprojection system. The reservoir is formed by a liquid confinementstructure 12 positioned below and surrounding the final element of theprojection system PL. Liquid is brought into the space below theprojection system and within the liquid confinement structure 12. Theliquid confinement structure 12 extends a little above the final elementof the projection system and the liquid level rises above the finalelement so that a buffer of liquid is provided. The liquid confinementstructure 12 has an inner periphery that at the upper end preferablyclosely conforms to the shape of the projection system or the finalelement thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular, though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air, synthetic air, N₂or an inert gas, provided under pressure via inlet 15 to the gap betweenliquid confinement structure 12 and substrate and extracted via outlet14. The overpressure on the gas inlet 15, vacuum level on the outlet 14and geometry of the gap are arranged so that there is a high-velocitygas flow inwards that confines the liquid.

A gap also exists under the substrate W, between the bottom surface ofthe substrate W and the top surface of a substrate holder (also known asa burl plate or pimple plate) located in an indent in the substratetable WT. This gap may also comprise a gas seal of sorts to keep thebottom surface of the substrate W dry and free from the energy lossproblems discussed herein.

A net energy loss is caused when more immersion liquid 11 evaporatesthan condenses. Localized energy fluctuations may cause distortion ofthe substrate or of the resist layer on the substrate. An embodiment ofthe present invention seeks to prevent localized energy fluctuations byreducing the evaporation rate, increasing the condensation rate, orboth.

To compensate for this temperature reduction and energy loss when netevaporation is greater than net condensation, there are several possiblesolutions.

A first solution is to supply humidified compressed clean gas (e.g. air)to the immersion system, for example, via the gas knife, in order toreduce the parasitic evaporation of the immersion liquid by reducing thedifference in relative humidity of the atmosphere inside the immersionsystem and the gas in the gas knife. Overlay errors and other problemslinked to the presence of an immersion liquid and liquid confinementstructure 12 may be tackled by a liquid evaporation controller, whichtargets and controls the rate of evaporation of immersion liquid in theregion of the substrate. Molecules of liquid absorb energy from thesurroundings in order to evaporate and, particularly if pumped away, thecooling that results can result in significant and non-uniformvariations in the temperature of critical components such as thesubstrate. Thermally-induced distortions can lead to errors in the imagefinally exposed to the substrate. For example, evaporation of immersionliquid left behind on the substrate after the liquid confinementstructure 12 has passed by can cause local temperature drops of up to 3degrees K. As a result of this, at least a 20 nm single machine overlayerror may typically result. However, this solution has a possibledrawback of not being terribly efficient because the requiredover-pressure of the gas in the liquid confinement system for the gasknife means that 100% relative humidity may not be achieved after theexpansion of the compressed gas. This is why evaporation nonetheless mayoccurs with this solution.

A second solution is thermal conditioning (i.e. supplying energy to thesubstrate table to reduce net energy loss) of the substrate W with aliquid flow to the substrate holder WH and/or the substrate table WT.The intention of this is to supply the evaporation energy with as smalla temperature change as possible compared to a idle system and therebyslow down the evaporation rate of the immersion liquid, reducing thermalimpact through, for example, thermo-mechanical deformations. Thesubstrate table should remain at the system reference temperature. Thesame problems occur with this solution in that the whole substrate iscooled down and localized temperature fluctuations may not be inhibited.

A third solution in which errors caused by the cooling effects ofevaporating immersion liquid are tackled is to provide a substrateheater configured to heat a portion of the substrate W according to theposition, velocity, acceleration and/or path of the substrate table WTrelative to the liquid confinement structure or the local substrateand/or substrate table temperature. Other parameters on which the heatercharacteristics may be dependent are the resist characteristics—inparticular, the contact angle of radiation on the resist and thesubstrate used for the resist. The substrate heater may provide heatingvia one or more of a number of mechanisms. These may include one or moreof the following: an infra-red emission source, a glow wire electricalresistance heater and a hot gas jet. Important factors when decidingwhich kind of heater to use include how finely and quickly the heatingpower needs to be adjusted, as well as how effectively the heater can beproduced in miniature form. The latter factor will become more or lessimportant depending on whether the heater needs to be embedded in ornear the material whose temperature it is intended to regulate (such asa glow wire, for example, embedded in the substrate holder and/orsubstrate table) or whether the heater works to some extent at adistance (such as a radiation-emitting source or temperature-controlledgas jet source). In the case of a radiation-emitting source, thewavelength distribution of the radiation should be chosen so as not toreact with resist compositions on the substrate (infra-red radiationwould be safe in this regard for most resists of interest). Selection ofradiation intensity will depend on the optical properties of the resist(such as its reflectivity). The heating system should be tuned for aprocess or have an adequate temperature sensing mechanism. There arelikely advantages and disadvantages with both—tuning may require a lesscomplex design, whereas temperature sensing may give a greater systemavailability than tuning.

As discussed above, one of the reasons more liquid may evaporate thancondense is that it is exposed to atmospheric conditions, for example,on the top surface of the substrate W or of the substrate table WT. FIG.6 shows the substrate table WT with respect to the liquid confinementstructure 12 and arrow 32 shows the direction in which the substratetable WT travels; i.e. laterally with respect to the liquid confinementstructure 12. In order to know at what locations evaporation is mostlikely to occur, in an embodiment, a timetable 34 is created whichcomprises the position, velocity, acceleration and/or path of thesubstrate table WT with respect to the liquid confinement structure 12relative to various points in time. Liquid evaporation controller 30 maybe used in conjunction with timetable 34. The liquid evaporationcontroller 30 may take several forms, as long as the net energyextraction due to evaporation is controlled (rather than the evaporationitself being controlled). For example, it may heat the gas 16 which issupplied via inlet 15 and removed by outlet 14 such that the such thatthermal equilibrium of the system remains constant, even as thesubstrate moves out from underneath the liquid confinement structure.Alternatively, the evaporation controller 30 may supply compressedhumidified gas 16 via the inlet 15 and outlet 14. In an embodiment, thegas may be any suitable gas, such as air and/or nitrogen (nitrogen maybe particularly useful when a liquid is used in the immersion systemthat cannot be exposed to air). This reduces the evaporation from thetop surface of substrate W, thus equilibrating evaporation andcondensation and thereby creating an energy equilibrium in the immersionsystem. With humidified gas, it is the evaporation that is suppressed,rather than the transfer of heat energy from the substrate to the gas.

An alternative or additional evaporation controller is shown in FIG. 7.In this embodiment, humidified or heated gas 16 may still be introducedvia the liquid confinement structure 12. Additionally or alternatively,a temperature conditioned liquid flow is supplied to the substrateholder. This lowers the temperature below the substrate W in thesubstrate table WT, thus increasing the rate of condensation at coolspots on the bottom surface of the substrate W because most vaporcondenses to liquid when it is cooled down. The energy from the vapor isthen transferred to the substrate (and other surfaces) on which theliquid has condensed. This increase in condensation on the bottom of thesubstrate and decrease in evaporation on the top of the substrate may befinely balanced in order to ensure that the substrate substantiallymaintains its thermal equilibrium, even when it has been moved out ofrange of protection of the liquid confinement structure 12.

A stable reference temperature is readily achieved by introducing a heatpipe 48 in the substrate holder WH, below the substrate W. Liquid (e.g.,water) flowing through the heat pipe 48 may be conditioned using atemperature and/or pressure sensor and a heater, maintaining asubstantially constant temperature in the pipe. Liquid vapor coming fromthe liquid in the heat pipe 48 will only condense at cold spots in thesubstrate table, transferring the latent heat from the vapor to the coldspot. No complex control mechanism is therefore required and atemperature overshoot is unlikely. A pressure sensor may be used toensure that the pressure of, for example, the liquid vapor is not toohigh. The pressure of the liquid vapor should be controlled, in anembodiment where the liquid is water, to a tolerance of better than 4 Pafor a condensation temperature stability of 20 mK.

Any gas, or other fluids other than the working fluid, in the heat pipe48 should be pumped out before the system is closed in order to optimizethe heat transfer from the liquid and liquid vapor to the cold spots inthe substrate holder. The heater that heats the liquid can be placeddirectly in the liquid or under the substrate table, thereby keeping thebottom part of the substrate table heated to a stable temperature, whichin turn keeps the enclosed liquid at a stable temperature.

To avoid splashing of the liquid, a sponge-like material can be placedin liquid grooves in the substrate table WT, around the substrate holderWH. The sponge-like material is also placed such that it removes theliquid from the condensation areas in order to make those locationsavailable for further vapor to condense.

Furthermore, the heat pipe 48 can be made up of several channels andthese channels can be positioned as close to the surface of thesubstrate holder WH as possible so that the temperature of the liquid inthe channels affects the temperature of the substrate W most directly.There can be hollow pimples or dimples in the substrate holder WH, belowthe substrate, and these pimples or dimples can contain passages thatallow liquid vapor to pass through to the underside of the substrate andcondense directly onto the cold spots of the substrate, rather than justat the cold spots of the substrate holder. In this way, the heattransfer is more direct and the time taken to transfer the heat isreduced, reducing the chance of temperature fluctuations.

Humidified gas (e.g., humidified air) that is introduced to the topsurface of the substrate is prone to expansion after the gas knife haspassed over the substrate. This causes a relative humidity of less than100%. In the area between the gas knife and a gas knife extractionconduit, the humidified gas cools down and its humidity increases againbecause colder gas has a lower humidity capacity than warm gas. Warm,humidified gas may therefore be supplied after the gas knife. In anembodiment, for a drop of 0.4 bar during the passing of the gas knife,water saturated air at approximately 28° C. may be used in order to givepost-gas knife air at 22° C. with 100% relative humidity. Evaporatedliquid in the warm gas is condensed out again after expansion to leave anull energy result on the substrate.

FIG. 8 shows an alternative or additional embodiment that may be used tocompensate for evaporation from the top of the substrate W. As thesubstrate W moves towards the right, in the direction of the arrow, theupper surface of the substrate W, which has been in contact with theimmersion liquid 11, undergoes drying by a gas knife 40. The gas knifesupplies a wedge of gas (e.g., air) to the surface of the substrate inorder to remove the final thin film of immersion liquid 11 remaining onthe substrate after the liquid confinement structure 12 has beenremoved. This is a common time at which the substrate will cool down andthereby lose energy. The typical loss of local cooling power caused bythe gas knife is of the order of 20 Watts on hydrophilic surfaces (e.g.at a 60° contact angle) when using air.

There are one or more ways to compensate for this evaporation energyloss using the gas knife. A first is to supply high pressure humidifiedgas (e.g., air) via the gas knife. Another is to heat the gas (e.g.,air) in the gas knife, for example using a radiation source 38 such asan LED or a microwave cavity with one or more conditioning tubes.

The embodiment shown in FIG. 8 has a temperature sensor 22 which inputsinformation into a controller (such as a CPU) 36, along with timetableinformation 34. When the substrate W is most likely to be losing energythrough evaporation caused by the gas knife 40, the CPU 36 controls theradiation source 38 in order to increase the local surface temperatureof the substrate W at that point that it is likely to be losing energy,thereby reestablishing the thermal equilibrium.

The wavelength range of the radiation source must be chosen so that itdoes not expose the resist on the surface of the substrate, and so thatthe light is absorbed by the same amount on all process layers. Multipleradiation sources may be arranged over the periphery of the gas knife sothat the section of the substrate where an “evaporation trail” is leftbehind may be compensated for effectively. Further information that maybe put into the timetable (which may also be known as a fast forwardtable) may be job layout data, scan speed and/or resist characteristics.The information supplied by the controller to the radiation source mayalso steer the direction in which the radiation source transmits itsradiation in order to more finely control the surface temperature of thesubstrate.

FIG. 9 shows another or alternative embodiment for increasingcondensation to the bottom of the substrate, thereby restoring thermalequilibrium in a different manner from heating the top. The embodimentsmay, of course, be combined to enhance the precision of the thermalequilibrium.

Supplying liquid saturated gas 44 (e.g. saturated ethanol atapproximately 24 mbar or 60 mbar) in the vacuum 42 between the substrateW and the substrate holder WH will lead to condensation of liquid (e.g.,water) at the cold spots of the substrate W. The energy loss bycondensation will heat the substrate gas temperature. As long assaturated gas 44 is supplied, the condensed liquid under the substrate Wwill not evaporate. To compensate for average energy losses at the topsurface of the substrate W, only a few micron thick liquid film isrequired to condense on the bottom surface of the substrate W.

The liquid on the under side of the substrate W is subsequently removedby a supply of dry gas after the last exposure of the substrate, beforethe substrate is unloaded. Liquid (e.g., water) evaporation underpressure is very quick. The advantage of this method of energyregulation is that the energy can be supplied at the exact location ofthe substrate W where energy loss is occurring without any activecontrol. This is because the liquid will condense at the coldest partsof the substrate W. An additional advantage is that a temperatureovershoot is not possible. This is because only the heat capacity of thesubstrate W participates in the transient behavior of the liquid andthus energy regulation is carried out in the shortest possible time. Inother words, as the substrate cools down through energy losses on itstop surface, liquid condenses on its bottom surface, thus transferringenergy back in as quickly as it escapes.

FIG. 10 shows a closing plate 46 supplying the bottom surface of theliquid confinement structure 12, for example, while a substrate is beingswapped. The position, speed, acceleration and timing of the closingplate may also be added into the information in the timetable 34.Conditioning the closing plate allows the immersion liquid to be moreeasily maintained at the same temperature/energy.

In an alternative or additional embodiment of the present invention,evaporation control methods are actively applied when a reduction intemperature of the substrate is detected. Changes in temperature may bedetected in the immersion liquid itself with a detector 22 as shown inFIG. 11; or the temperature sensor may be in different positions aroundthe substrate W itself, such as in one or more of the positions shown inFIG. 12. Temperature sensor 22 a hovers above the substrate W and FIG.12 shows a lead 24 connecting the temperature sensor to a CPU, such asthat shown in FIG. 8 (CPU 36). Temperature sensor 22 b is inside thesubstrate W and sensor 22C is in the vacuum 42 between the substrate Wand the substrate holder WH. The scale of the vacuum 42 with respect tothe substrate W and the substrate holder WH has been exaggerated forillustrative purposes. Sensors may also be placed in the liquid supplysystem. Temperature sensors may be positioned in one or a plurality ofthese positions in order to sense the temperature of the substrate indifferent places, depending on the likely cause of the loss of energy atthat position. A sensor may further be placed in the substrate tableitself, though it would be sensible to have it in a position such asthat shown in FIG. 12 and labeled as 22 c so that it is as close to thesurface of the substrate table and/or the surface of the substrate W aspossible because it is the substrate that may suffer if the thermalequilibrium of the substrate is lost.

A heating element may be simultaneously switched with the use ofhumidified compressed clean gas (e.g., air), for example, as describedwith respect to FIG. 6. In this way, the evaporation process iscompensated over time by the energy dissipation of the heater withoutthe need of a temperature control system. Evaporation of the liquid filmon the substrate takes place when the liquid confinement system moves(e.g., 25 W energy loss when the substrate is moving compared to a 5 Wenergy loss when it is standing still in a system using liquid and air).The rate of movement affects the rate of the energy loss. The liquidfilm is left on the substrate behind the gas knife, rather than in frontand so the information on the movement pattern and speed of thesubstrate with respect to the immersion system is used to switch boththe power of the compensation and the location of the substrate withrespect to the liquid confinement system in the immersion system. Resistparameters (e.g. wettability by the immersion liquid) and gas knife gasparameters (e.g. Marangoni effect) determine the required heating power.

When using a passive system, such as that described with respect to FIG.9, with no temperature sensor 22, there is no control loop and so veryfew electronics are required. It is therefore easy to adapt existingmachines if required, although new machines may of course be built withintegrated temperature compensation systems.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus configured to project a patterned radiationbeam onto a target portion of a substrate held on a substrate holder ofa substrate table, comprising: a liquid supply system configured to atleast partly fill a space between a projection system and the substratetable with liquid; a liquid confinement structure arranged to at leastpartially contain the liquid within the space; a substrate tabledisplacement system arranged to move the substrate table along a pathrelative to the liquid confinement structure, thereby to move the targetportion over the surface of the substrate; a timetable comprisingpositions and timings of the substrate displacement system at which thesubstrate is likely to be undergoing localized cooling; and a liquidevaporation or condensation controller arranged to control the net rateof energy lost through evaporation or condensation of liquid supplied bythe liquid supply system by synchronizing evaporation or condensationcontrolling techniques with the timetable.
 2. The apparatus according toclaim 1, wherein the evaporation controller comprises a substrate heaterconfigured to heat a portion of the substrate according to the (i)position, (ii) velocity, (iii) acceleration, (iv) path, or (v) anycombination of (i)-(iv) of the substrate table relative to the liquidconfinement structure, the local substrate temperature, or the localsubstrate table temperature.
 3. The apparatus according to claim 1,wherein the evaporation controller comprises a substrate table heaterconfigured to heat a portion of the substrate table according to the (i)position, (ii) velocity, (iii) acceleration, (iv) path, or (v) anycombination of (i)-(iv) of the substrate table relative to the liquidconfinement structure, the local substrate temperature, or the localsubstrate table temperature.
 4. The apparatus according to claim 1,wherein the evaporation controller is configured to thermally conditionthe substrate using a liquid flow to the substrate table.
 5. Theapparatus according to claim 1, wherein the evaporation controller isconfigured to thermally condition the substrate table using a liquidflow to the substrate table.
 6. The apparatus according to claim 1,wherein the evaporation controller is configured to control atemperature of the substrate table by the incorporation of a volumecontaining liquid and vapor in the substrate holder.
 7. The apparatusaccording to claim 6, wherein the volume is a pipe.
 8. The apparatusaccording to claim 6, wherein the liquid is water and the vapor is watervapor.
 9. The apparatus according to claim 6, wherein the liquid is ahydrocarbon containing refrigerant and the vapor is a hydrocarbon vapor.10. The apparatus according to claim 6, wherein the liquid is R134a,1,1,1,2-tetrafluoroetane.
 11. The apparatus according to claim 6,wherein the liquid is a mixture of hydrocarbon containing refrigerantsand the vapor is a mixture of hydrocarbon vapors.
 12. The apparatusaccording to claim 11, wherein the liquid is R401a.
 13. The apparatusaccording to claim 6, wherein the liquid is ammonia and the vapor isammonia vapor.
 14. The apparatus according to claim 6, comprising aheater configured to heat the liquid.
 15. The apparatus according toclaim 14, wherein the heater is configured to be located in the liquid.16. The apparatus according to claim 14, wherein the heater ispositioned below the substrate table.
 17. The apparatus according toclaim 14, further comprising a temperature and/or pressure sensor toprovide an input parameter for the heater.
 18. The apparatus accordingto claim 6, wherein the volume is empty of gas and non-condensablefluids, and is sealed from the atmosphere.
 19. The apparatus accordingto claim 6, wherein the volume comprises a plurality of channels, whichare as close to a substrate holder surface configured to face thesubstrate as possible.
 20. The apparatus according to claim 6,comprising hollow dimples in a surface of the substrate holderconfigured to face the substrate.
 21. The apparatus according to claim20, wherein the hollow dimples comprise passages between the volume andthe substrate such that they allow vapor to pass from the volume to abottom surface of the substrate.
 22. The apparatus according to claim 6,further comprising a sponge-like material in the substrate table. 23.The apparatus according to claim 1, further comprising a gas knifeconfigured to supply a stream of gas to the substrate surface betweenprojections by the projection system, wherein the evaporationcontrolling technique comprises supplying compressed humidified gas inthe gas knife.
 24. The apparatus according to claim 1, wherein theevaporation controlling technique comprises supplying compressedhumidified gas into the liquid confinement system.
 25. The apparatusaccording to claim 1, further comprising a gas knife configured tosupply a stream of gas to the substrate surface between projections bythe projection system, wherein the evaporation controlling techniquecomprises heating of the gas in the gas knife.
 26. The apparatusaccording to claim 25, configured to heat the gas in the gas knife usinga light emitting diode.
 27. The apparatus according to claim 25,configured to supply warm humidified gas to an area between the gasknife and a gas knife extraction conduit.
 28. The apparatus according toclaim 27, further comprising a condensation separator configured toseparate condensation from the warm humidified gas upstream of theliquid confinement structure.
 29. The apparatus according to claim 1,configured to have a vacuum between the substrate holder and thesubstrate and to supply liquid-saturated gas into the vacuum.
 30. Theapparatus according to claim 1, wherein the timetable comprises timeswhen the substrate is likely to be cooled and therefore when evaporationof liquid from the substrate is likely to occur.
 31. The apparatusaccording to claim 30, wherein the timetable further comprisesevaporation power and location on the substrate table or substrate as afunction of time.
 32. The apparatus according to claim 30, furthercomprising a gas knife configured to supply a stream of gas to thesubstrate surface between projections by the projection system, whereinthe timetable comprises times and positions of the gas knife over thesubstrate as a time the substrate is likely to be cooled.
 33. Theapparatus according to claim 30, wherein the timetable further comprisesthe sequence number of a present substrate in a sequence of substrates.34. The apparatus according to claim 30, wherein the timetable furthercomprises input from a sensor comparing actual substrate timing withprecalculated optimal timing.
 35. The apparatus according to claim 30,wherein the timetable further comprises input regarding a parameter ofthe radiation irradiating the substrate.
 36. The apparatus according toclaim 30, further comprising a closing plate configured to keep theliquid within the liquid confinement structure when the liquidconfinement structure is not over the substrate, wherein the timetablecomprises the times when the closing plate is not containing the liquidas a time the substrate is likely to be cooled.
 37. The apparatusaccording to claim 1, wherein the timetable comprises positions on thesubstrate where the substrate is likely to be cooled and therefore fromwhere evaporation of liquid from the substrate is likely to occur. 38.The apparatus according to claim 37, wherein the timetable furthercomprises evaporation power and location on the substrate table orsubstrate as a function of time.
 39. The apparatus according to claim37, wherein the timetable comprises the relative position of thesubstrate with respect to the liquid confinement structure.
 40. Theapparatus according to claim 37, wherein the timetable comprises theposition of the top surface of the substrate.
 41. The apparatusaccording to claim 37, wherein the timetable comprises the position ofthe bottom surface of the substrate.
 42. The apparatus according toclaim 37, wherein the timetable comprises the position of an exhaustchannel of the liquid confinement structure.
 43. The apparatus accordingto claim 1, further comprising a desorption device configured to desorbgas from the substrate table that has been adsorbed during contact withthe liquid supply system.
 44. The apparatus according to claim 1,further comprising a regeneration device configured to return to itsoriginal phase any part of the substrate table that has changed phaseduring contact with the liquid supply system.
 45. The apparatusaccording to claim 1, wherein the liquid evaporation controllercomprises a temperature sensor.
 46. The apparatus according to claim 45,wherein the temperature sensor is in the liquid confinement structure.47. The apparatus according to claim 45, wherein the temperature sensoris configured to hover above the substrate.
 48. The apparatus accordingto claim 45, wherein the temperature sensor is configured to be betweenthe substrate and the substrate table.
 49. The apparatus according toclaim 45, wherein the temperature sensor is in the substrate table. 50.A device manufacturing method, comprising: supplying a liquid to a spacebetween a projection system of a lithographic apparatus and a substrateusing a liquid supply system; controlling a net rate of energy lostthrough evaporation or condensation of liquid supplied by the liquidsupply system by synchronizing evaporation or condensation with atimetable comprising positions and timings of the substrate at which thesubstrate is likely to be undergoing localized cooling; and projecting apatterned radiation beam through the liquid onto a target portion of thesubstrate.